<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            padding: 120px;
        }

        .mainbox {
            width: 320px;
            height: 320px;
            position: relative;
            /* 超出隐藏需要加上 */
            overflow: hidden;
        }

        .content {
            width: 320px;
            height: 320px;
            line-height: 320px;
            text-align: center;
            background-color: #cde;
        }

        .line {
            /* 结合外层元素的相对定位 */
            position: absolute;
        }

        .line:nth-child(1) {
            top: 0;
            left: 0;
            width: 100%;
            height: 3px;
            /* 加上渐变效果，方可形成拖尾效果 */
            background: linear-gradient(90deg, transparent, orange);
            animation: animate1 8s linear infinite;
        }

        /* 分别控制其上下左右的定位距离，从而形成线条跟随效果 */
        @keyframes animate1 {
            0% {
                left: -100%;
            }

            50%,
            100% {
                left: 100%;
            }
        }

        .line:nth-child(2) {
            top: -100%;
            right: 0;
            width: 3px;
            height: 100%;
            background: linear-gradient(180deg, transparent, red);
            animation: animate2 8s linear infinite;
            /* 注意要加上延时触发动画效果，这样线条才会依次触发 */
            animation-delay: 2s;
        }

        @keyframes animate2 {
            0% {
                top: -100%;
            }

            50%,
            100% {
                top: 100%;
            }
        }

        .line:nth-child(3) {
            bottom: 0;
            right: 0;
            width: 100%;
            background: linear-gradient(270deg, transparent, green);
            animation: animate3 8s linear infinite;
            animation-delay: 4s;
        }

        @keyframes animate3 {
            0% {
                right: -100%;
                height: 3px;
            }

            50%,
            100% {
                height: 2px;
                right: 100%;
            }
        }

        .line:nth-child(4) {
            bottom: -100%;
            left: 0;
            width: 3px;
            height: 100%;
            background: linear-gradient(360deg, transparent, #3a86ff);
            animation: animate4 8s linear infinite;
            animation-delay: 6s;
        }

        @keyframes animate4 {
            0% {
                bottom: -100%;
            }

            50%,
            100% {
                bottom: 100%;
            }
        }
    </style>
</head>

<body>
    <div class="mainbox">
        <span class="line"></span>
        <span class="line"></span>
        <span class="line"></span>
        <span class="line"></span>
        <div class="content">线条环绕</div>
    </div>
</body>

</html>